fix(wopi): improve robustness and error handling in getUrlSrc

[joshtrichards]

• Resolves: #
• Closes Exception":"Error","Message":"Call to a member function xpath() on bool #3895
• Target version: main

Summary

This PR refactors the getUrlSrc() method in the WOPI Parser to improve its robustness, maintainability, and readability:

• Adds early error returns for null, empty, or malformed discovery XML.
• Improves attribute checks by validating that both urlsrc and name are present and non-empty in the action node.
• Provides detailed logging for each error scenario, including when action nodes or expected attributes are missing.
• Ensures consistent exception messages for each failure path.
• Maintains a clear control flow and reduces nesting by handling errors as soon as they are detected.

These changes improve reliability when parsing WOPI discovery responses and make troubleshooting easier.

TODO

• ...

Checklist

• Code is properly formatted
• Sign-off message is added to all commits
• Documentation (manuals or wiki) has been updated or is not required

[joshtrichards]

I guess we should decide whether we should throw a specific exception if the issue is merely that there isn't a matching app name -- or just an empty array and let callers check for that...